The present invention relates to an electrophotographic apparatus comprising an improved thermal fixing means.
After a photoconductive drum of an electrostatic copying machine is exposed to a light image of an original document thereby forming an electrostatic image thereon, a toner substance is applied to the drum to develop the electrostatic image into a toner image. The toner image is then transferred and fixed to a sheet of copy paper to provide a permanent reproduction of the original document.
Fixing is generally accomplished by means of heat, pressure or a combination thereof. The present invention relates to the type of fixing means which utilizes electric heaters to radiate heat onto the copy sheet to fix the toner image thereto by means of fusion.
It has heretofore remained a problem to maintain the heaters at the proper constant temperature. One expedient which has been proposed is to sense the temperature in the fixing means utilizing a thermistor or similar electronic heat sensor and alternately turn the heaters on and off in accordance with the output signal from the thermistor to maintain the temperature at a predetermined value. While such a system generally works, the thermistor and comparator circuitry required are costly and tend to malfunction often. Also, the response time of the heaters is too long to effect precise temperature control.
Another prior art expedient has been to provide the heaters with a high thermal coefficient, thereby minimizing the flucuations in temperature caused by varying thermal load due to the number of copies produced and other factors. While this expedient does help somewhat, it leads to substantially increased power consumption.
Another expedient has been to energize the heaters constantly at a level of power such as to maintain the heaters at the required temperature. In addition to being sensitive to variations in thermal load, this expedient involves a substantial waste of power since the fixing means only needs to be maintained at its operating temperature when a copy sheet is actually passing therethrough.